No, that's not true,Because the last dynasty in Viet Nam was Nguyen so, many peoples changed their last name to Nguyen, why ? I don't know. I am looking for my real last name too,But I can give you one example why it's not true,My high school class had 25 students, 13 of them had this last name, included me. There are also some others last name too, like Phan, Vo, Vu, Dam, Vuong, Dang, Do, Doan, ...